Breast cancer is a disease that kills over 45,000 women each year in the United States alone. Over 180,000 new cases of breast cancer are diagnosed annually, and it is estimated that one in eight women will develop breast cancer. These numbers indicate that breast cancer is one of the most dangerous diseases facing women today. Cancer research has been unable to determine the cause of breast cancer, and has not found a suitable method of therapy or prevention.
The standard of care currently includes screening the tumor for the expression levels of the hormone receptors, estrogen receptor (ER) and progesterone receptor (PR), and the human epidermal growth factor receptor 2 (HER2) kinase. Currently, a woman diagnosed with breast cancer may be treated preliminarily with surgery, chemotherapy (optional in some cases), and radiation before targeted therapy is initiated. Hormone receptor-positive breast cancers are susceptible to hormone therapies with selective estrogen receptor modulators or SERMs (e.g., tamoxifen, toremifene), aromatase inhibitors (e.g., anastrozole), or selective estrogen receptor degraders or SERDs (e.g., fulvestrant). Hormone therapies such as aromatase inhibitors (AI) block production of estrogens in the body (typically used in post-menopausal women), whereas SERMs and SERDs block the proliferative action of estrogens on the breast cancer cells. Hormone receptor-positive and HER2-negative patients that have failed initial hormone therapy are often treated with an inhibitor of cyclin-dependent kinase (CDK) types 4 and 6 (CDK4/6 inhibitor) palbociclib (Ibrance) in combination with either letrozole or fulvestrant. HER2-positive breast cancers are susceptible to HER2 kinase inhibitors (e.g., trastuzumab and lapatinib) and are generally used in metastatic disease. Anti-angiogenic therapy (bevacizumab) is also approved in metastatic disease. Despite these multiple tiers of targeted treatments, patients often have or develop refractory forms of breast cancer. Examples of refractory breast cancer include primary tumors which are triple-negative (lacking ER, PR, HER2), hormone resistant (SERM-, SERD-, or AI-resistant), or kinase inhibitor resistant, or metastatic breast cancer tumors. Once the targeted therapies fail or tumors metastasize, radiation and high dose chemotherapy are required to ablate the refractory breast cancer tumors. Current chemotherapies available for the treatment of refractory breast cancer include anthracyclines, taxanes, and epothilones, which are toxic, dangerous, costly, and often are ineffective, especially in the treatment of metastatic disease.
Abundant clinical evidence suggests that androgens normally inhibit breast growth. For instance, women with androgen deficits have an increased risk for developing breast cancer. Androgen signaling plays a crucial role in breast homeostasis, negating the proliferative effects of estrogen signaling in the breast. However, when androgens transform into estrogens (aromatase pathway), they increase cell proliferation and mammary carcinogenesis risk. Historically, the steroidal androgen receptor agonists testosterone, fluoxymesterone, and calusterone were used in advanced breast cancer. These agents suffered from side effects such as excessive virilization, cross-reactivity with the estrogen receptor, and aromatization to estrogens. The use of steroidal androgens in advanced breast cancer pre-dates the screening of breast cancers for hormone and kinase receptors. Recently, it was found that the AR is expressed in 50-90% of breast tumors, providing a mechanism to use androgens as targeted therapy for AR-positive breast cancers.
Although the majority of breast cancers are considered hormone receptor-positive (ER, PR, or HER2), 15-20% of women diagnosed with breast cancer will have Triple Negative Breast Cancer (TNBC) which is characterized by a lack of expression of ER, PR, or HER2. TNBC occurs more frequently in younger patients (<50 years of age) and generally shows a more aggressive behavior. For those patients with advanced TNBC, standard palliative treatment options are limited to cytotoxic chemotherapy. However, even after initial response to chemotherapy, the duration of the response may be short and there is a higher likelihood of visceral metastases, rapidly progressive disease, and inferior survival compared to hormone positive breast cancer. Therefore, research is focused on identifying therapeutic targets in TNBC. One such target is the androgen receptor (AR). The AR is the most highly expressed steroid receptor in breast cancer with up to 95% of ER-positive breast cancers expressing AR (see Example 9 infra). In TNBC, up to 30% of cancers may express AR. Historically, AR has been considered anti-proliferative and beneficial in hormone positive breast cancers. In TNBC, data demonstrates that the presence of AR and androgen synthesizing enzymes is associated with lower proliferation, lower tumor grade, better overall survival, and more favorable clinical outcomes as compared to those patients with TNBC not expressing AR. Evidence also suggests that the AR target gene prostate specific antigen (PSA) is a favorable prognostic marker in breast cancer. Based on these findings, research is focused on AR as a potential therapeutic target.
Selective androgen receptor modulators (SARMs) are compounds which demonstrate AR-mediated tissue selective activity. Unlike their steroidal precursors, SARMs are non-aromatizable, generally demonstrate no activity at other steroidal receptors including ER and PR, and are non-virilizing. Further, SARMs may be beneficial in refractory breast cancer patients due to their hypermyoanabolic effects that should improve their tolerance of high-dose chemotherapy.
New innovative approaches are urgently needed at both the basic science and clinical levels to develop compounds which are useful for: a) treating a subject suffering from breast cancer; b) treating a subject suffering from metastatic breast cancer; c) treating a subject suffering from refractory breast cancer; d) treating a subject suffering from AR-positive breast cancer; e) treating a subject suffering from AR-positive refractory breast cancer; f) treating a subject suffering from AR-positive metastatic breast cancer; g) treating a subject suffering from AR-positive and ER-positive breast cancer; and/or h) treating, preventing, suppressing or inhibiting metastasis in a subject suffering from breast cancer.